1. Field of the Invention
This invention relates to a heater control device to control temperatures of an injection cylinder, an injection nozzle, and so forth in an electrically-operated injection molding machine.
2. Description of the Related Art
AC current is supplied from an AC power source to a heater in an injection molding machine. Hence, temperature of the heater is controlled by a contactor or by a solid-state relay.
In a case of control by the contactor, an ON-OFF switching cycle is mechanically limited by a time required for a switching operation. It is thereby impossible to reduce an ON time interval and an OFF time interval. An increase in the ON time interval or the OFF time interval causes a problem in that a heater temperature to be controlled overshoots or undershoots a target value to a considerable extent. As a result, accurate control of the temperature becomes more difficult.
On the other hand, in case of control by the solid-state relay, the heater is turned ON or OFF according to voltage so that current and power can not easily be controlled. As a result, there is caused a problem in that a heat value of the heater can not accurately be controlled.
Further, when the AC power source is employed, supply current is varied to describe a sine curve in process of time. Therefore, constant current can not be supplied by making or breaking contact, or turning ON or OFF the solid-state relay, resulting in an irregular heat value.
As seen from the above facts, if the temperature of the heater is controlled by using the AC power source, irregularity in power supply extends a time required to increase the temperature. Further, fine control of current, voltage, or power becomes more difficult.